Cylindrical pressure vessels are used for a variety of industrial applications, and a family of relatively lightweight, strong, reinforced plastic composite pressure vessels has grown up as a result of advancements in synthetic fibers and polymeric resins, for example, using continuous strands of glass or carbon fibers or filaments to reinforce curable polymeric resin compositions, such as polyester resins, vinylester resins, polyurethane resins, epoxy resins and the like. Although there are many industrial uses for such pressure vessels, one major field of use is the field of filtration, both direct or dead end filtration and cross flow filtration; and for purposes of this application, filtration is used to broadly include separation treatments where semipermeable membranes are employed. "Filter cartridge" is used to broadly include filter bags, cartridge filters and semipermeable membrane cartridges or elements. "Cartridge filtration" is used to include such filtration operations using such filter cartridges.
Generally, "cross flow" filtration is contrasted with what is sometimes termed direct or "dead end" filtration. In cross flow filtration, only a portion of the feed liquid passes through the filter medium with the remainder of the feed liquid flowing across a membrane or other filter medium surface and exiting the other end of the filter cartridge; in such arrangement there are two exit ports from the pressure vessel, i.e. separate ports through which the cross flow concentrate and the filtrate or permeate exit. In direct or "dead end" filtration, the entire flow of the feed liquid passes through the filter media, and there is usually only a single exit stream.
Examples of cross flow filtration processes commonly employed in the last several decades include, but are not limited to, hyperfiltration (reverse osmosis), nanofiltration and ultrafiltration, all of which employ semipermeable membrane materials and are commonly referred to as membrane separation processes. In such operations, cylindrical cartridges have been conventionally employed which are received in a tubular surrounding pressure vessel having appropriate inlet and outlet ports. One or more end closures for such a pressure vessel are designed to have what is termed full-bore access; this allows a cylindrical cartridge or a bag having a defined exterior diameter just less than that of the interior bore of the pressure vessel to be slidably inserted through one open end thereof. In some instances, such cylindrical cartridges comprise a plurality of envelopes made of sheets of semipermeable membrane material which are spirally wound about a central porous core to provide a relatively large amount of membrane surface area within a given volume.
Many prior art pressure vessels used in cartridge filtration operations, especially cross flow filtration, have employed tubular housings made of glass fiber-reinforced polymeric resin composites provided with plug-type end closures, which are retained in sealing relationship at the end of the housing by a conventional O-ring seal and by appropriate retaining rings. Retaining rings which have been used include conventional spiral and snap rings, as well as segmental rings that comprise a plurality of separate pieces which are appropriately retained in assembled fashion by screws or like connectors. Generally in such reinforced-plastic composite pressure vessels, all the inlet and outlet ports have been provided in the pair of opposite end plugs, in order to avoid breaching the integrity of the cured composite body. The end plugs themselves are generally flat, plate-like bodies.
Safety considerations are frequently of prime importance in pressure filtration vessels, as many such filtrations require relatively high working pressures. Examples of filtration operations wherein it is important to be able to supply a feed at high pressure to the cartridges include those where semipermeable membrane permeation is being carried out, because such separation requires relatively high pressure difference across the membrane to achieve efficient operation.
Published Patent Application WO 88/03830 discloses closure assemblies for pressure vessels that can be used in cross flow or other pressure filtration equipment in order to provide full-bore access. Illustrated is an end plug having a cylindrical outer surface which can be slidably inserted within a smooth interior opening at the end of a pressure vessel. The end plug is provided with an axially inner female connector which receives a spigot at one end of a filtration element. When the end closure is installed, a pair of O-rings residing in an outer annular recess in the plug are physically compressed. The plug is secured in place by a locking element which seats in a groove within the pressure vessel interior cylindrical wall where it is secured in place by flanged keepers mounted by threaded bolts.
U.S. Pat. No. 4,781,830 shows a different type of end closure assembly for a cross flow filtration apparatus which also provides full-bore access. In the illustrated arrangement, an elongated cylindrical stainless steel housing is constructed which includes a generally bell-shaped adapter at each end that can include a side conduit connecting elbow. An end cap interfits with the adapter, via a bayonet-type fitting, to close the end opening, and a conventional O-ring is used to form a seal between the end cap and the seating surface on the adapter. This arrangement requires several specialized metallic subassemblies and is not considered to be particularly suitable for employment in a reinforced plastic composite pressure vessel arrangement.
Accordingly, improved versions of end closure assemblies for pressure vessels for pressure filtration, particularly fiber-reinforced polymeric resin vessels for cartridge filtration, have continued to be sought.